It is known that a polyorganosilane is useful as an electrically conductive organic material, a nonlinear optical material, a photodecomposable reaction initiator, a silicon carbide precursor, a photoresist material or the like (see Miller, R. D.; Michl. J., Chem. Rev., 89, 1359 (1989)).
The Wurtz method using an organohalosilane or organohalodisilane as a starting material has been widely used as a process for the preparation of a polyorganosilane. However, the Wurtz method is disadvantageous in that the use of metallic sodium or metallic potassium, which are self-igniting, in the air involves some risk, the reaction conditions are too severe to control the molecular weight of the product, and a polyorganosilane having a bimodal distribution of molecular weight is obtained.
In order to overcome these disadvantages, some approaches have been attempted. For example, Shono et al. obtained silicone polymers having organic groups comprising atoms and atomic groups other than silicon incorporated in the main chain having weight-average molecular weight of 5,000 to 18,000 by an electrode reaction using bis(halosilyl) compounds as starting materials (JP-A-4-348128 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")). Asuke et al. obtained a silicon polymer having a benzene ring incorporated in its main chain by reacting dichlorosilane with dilithiobenzene in a proportion of 1:1 (JP-A-4-342726). Ishikawa et al. obtained an electrically conductive polymer having a thienyl group incorporated in its main chain by polymerizing a bis(5-halogen magnesium thiophene) silane derivative in the presence of a nickel catalyst (JP-A-4-218533).
Further, Ishikawa et al. obtained a polymer having ethynylene incorporated in its main chain by allowing a cyclic silane derivative containing ethynylene to undergo ring opening polymerization (Organometallics 1992. 11. 1604-1618). Kashizaki et al. obtained a polysilane copolymer soluble in an organic solvent by conducting the Wurtz reaction of a dichlorosilane monomer with an .alpha.,.alpha.'-dichloroxylene derivative. Yamashita et al. obtained silicon polymers having quinone incorporated in an Si--Si bond by reacting disilanylene polymers with quinones (Macromolecules 1993. 26. 2143-2144).
The disproportionation reaction using an alkoxydisilane is advantageous in that the reaction can be effected under mild conditions without using metallic sodium or metallic potassium. Up to this date, the disproportionation reaction has provided various polyorganosilanes. However, taking into account the utilization in a nonlinear optical material, a photoresist, particularly to an electrically conductive material, etc., it is proposed that by employing a branched or network structure rather than the above-described straight-chain polyorganosilanes, the HOMO-LUMO energy band gap of the polymer is narrowed, enhancing the usefulness as an electrically conductive material. Further, the content of the three-dimensional element in the structure can be raised to enhance the heat resistance of the polymer itself. For such utilization, easy synthesis of silicon polymers having a greater variety of branched or network structures has been desired.
It is possible that such a variety of polyorganosilanes and copolymers are obtained by introducing desired organic groups into silane derivatives as starting materials so that they undergo reaction as conducted by Ishikawa et al. above. However, when such silane derivatives are obtained, difficulties are found. For example, the undesirable reaction of the organic group to be introduced into the silane derivative makes it difficult to selectively introduce the organic group into the silane derivative. Further, the resulting silane derivative can be hardly purified. Moreover, since the reactions proposed by Asuke et al. and Kashizaki et al. require the use of a large amount of metallic lithium or metallic sodium, the mass production of these organosilicon polymers on an industrial basis is difficult.
On the other hand, although chlorosilanes having various organic groups are easily available, methods for obtaining polyorganosilanes from such chlorosilanes in a safer and easier manner by utilizing a catalytic reaction are not known.